The present invention relates to a method and an apparatus for treating boron-containing waste. More particularly, the invention relates to a method for treating medium to low level boron-containing radioactive waste which occurs as a result of operation, modification or dismantling of a nuclear power plant or the like, and an apparatus for treating boron-containing radioactive waste.
Boron-containing waste is discharged from facilities which handle various types of radioactive substances, such as pressurized water reactor (PWR) type power plants. Since such waste (e.g., concentrated waste liquid) contains boric acid containing a radioactive nuclide, its components need to be finally fixed by cement or the like. However, boric acid hinders solidification of cement, thus restricting the amount of boric acid which can be added into cement. For example, the weight of boric acid that can be solidified per cement solidification product with a volume of 200 liters is only about 10 kg, thus posing the problem of an enormous amount of the solidified waste. The mechanism of hindrance to cement solidification by boric acid may be that Ca, a main component of cement, reacts with boric acid in the waste liquid to form a calcium borate film, which covers the surface of cement particles, thereby terminating the progress of a hydration reaction.
A method proposed to decrease the amount of solidified waste is to insolubilize boric acid contained in the concentrated waste liquid so that the boric acid will not react when mixed with cement. An example is to add a calcium compound into the concentrated waste liquid to convert boric acid into insoluble calcium borate. Japanese Unexamined Patent Publication No. 59-12399 discloses a method which comprises adding a calcium compound, such as calcium hydroxide, to a concentrated waste liquid of about 70xc2x0 C., aging the mixture at a temperature of 40xc2x0 C. or lower to precipitate particles of calcium borate, separating the precipitated particles, and solidifying the liquid with cement. This method can reduce the amount of the resulting solidified waste to one-third to one-seventh of the corresponding amount produced by mere mixing of a concentrated waste liquid with cement for solidification. Japanese Unexamined Patent Publication No. 59-18498 discloses a method of drying the aged precipitate by a dryer to form a powder, and solidifying the powder. Japanese Unexamined Patent Publication No. 10-132997 discloses a method comprising powdering a high temperature slurry of concentrated waste liquid by means of a dryer to increase the strength of the resulting cement solidification product and decrease the amount of solidified waste. A proposal has been made, therefore, for the use of a vertical thin film dryer having a very high efficiency of heat transfer and capable of simplified instrument layout.
However, these methods involve the problem that a waste liquid line (especially, a curved portion of piping leading to the dryer, flowmeter, pumped portion, connectors) is clogged due to the sedimentation and buildup of calcium borate. In kneading precipitated calcium borate with cement, it is desired to apply a method of kneading in a radioactive waste solidifying reservoir, so-called in-drum mixing, from the point of view of reducing equipment cost and operating cost. Because of poor kneading due to high viscosity of the kneaded materials, however, powder remains unchanged, thus making this method unfeasible. Even if a driving force is increased in in-drum mixing to enhance a kneading power, kneaded materials are accommodated to a capacity of only 60 to 70% of a storage reservoir, out of considerations for scattering and vorticity of the kneaded materials. As a result, the amount of waste formed increases. Hence, there was no choice but to use a dedicated kneader, thereby increasing equipment cost and operating cost.
The present invention aims to solve the above-mentioned problems. It is an object of this invention to provide a method and an apparatus for treating a boron-containing waste liquid, the method and apparatus preventing clogging of a waste liquid line and achieving an increase in feeding of a waste liquid. It is another object of the invention to provide a method and an apparatus for treating a boron-containing powder, the method and apparatus making in-drum mixing feasible by lowering viscosity of a boron-containing powder and a cementitious powder when kneaded, and the method and apparatus increasing the strength of a cement solidification product.
The inventors of this invention found that clogging of a waste liquid line could be prevented and the feedability of a boron-containing waste liquid to a dryer could be improved, by adding not only an alkali earth metal element compound, but also an alkali metal element compound to the waste liquid, and maintaining the mixture at a high temperature. This finding led them to accomplish the invention. The inventors also found that a boron-containing powder obtained by a method, such as the method of the invention, could lower viscosity during kneading with a cementitious powder containing a chemical admixture, make in-drum mixing feasible, and enhance the strength of a cement solidification product. This finding led to the invention.
Therefore, a method for treating a boron-containing waste liquid according to the invention comprises the steps of adding an alkali metal element compound, and an alkali earth metal element compound to the boron-containing waste liquid, and then drying and powdering the boron-containing waste liquid, wherein a temperature of the boron-containing waste liquid during a period from addition of either the alkali metal element compound or the alkali earth metal element compound until the drying and powdering is maintained at not lower than a precipitation temperature of a compound containing boron and an alkali metal and a compound containing boron and an alkali earth metal.
A method for treating boron-containing waste according to the invention is characterized by kneading and solidifying a powder containing boron, an alkali metal and an alkali earth metal, a cementitious solidifying material, a chemical admixture for improving fluidity of cement, and water.
An apparatus for treating a boron-containing waste liquid according to the invention comprises means for storing the boron-containing waste liquid, means for supplying an alkali metal element compound to the boron-containing waste liquid, means for supplying an alkali earth metal element compound to the boron-containing waste liquid, means for drying and powdering the boron-containing waste liquid, and means for adjusting a temperature of the boron-containing waste liquid.
An apparatus for treating boron-containing waste according to the invention comprises means for supplying a powder containing boron, an alkali metal, and an alkali earth metal to a kneading container, means for supplying a cementitious solidifying material to the kneading container, means for supplying a chemical admixture for improving fluidity of cement to the kneading container, means for supplying water to the kneading container, and means for kneading the boron-containing powder, the cementitious solidifying material, the chemical admixture, and the water in the kneading container.